(CNN) -- Seven Blackwater USA employees, all Americans, died Thursday in Iraq, the company said.

The deaths bring the number of Blackwater employees killed in Iraq to 18.

Six employees of the American security contractor were killed Thursday when a Bulgarian commercial helicopter crashed north of Baghdad.

A seventh died when a roadside bomb detonated next to one of the company's armored personnel carriers near Ramadi. Four Blackwater employees were wounded in the Ramadi attack. All were working under contract to the U.S. military.

"This is a very sad day for the Blackwater family," Blackwater USA President Gary Jackson said in a statement. "We lost a number of our friends to attacks by terrorists in Iraq and our thoughts and prayers go out to their family members."

On March 31, 2004, four Americans, all Blackwater contractors, were murdered in Falluja, and the bodies of two of them were hung from a bridge. That incident escalated the conflict between U.S. forces and the insurgency.

Blackwater is one of a number of private security contractors that hire veterans for jobs previously assigned to the military.

Those jobs include the protection of personnel working for private companies and nongovernmental organizations in Iraq.

"They provide very focused security for detailing out how a protectee's day occurs -- from the beginning of the morning until they tuck that person back into bed at night," said CNN national security analyst Ken Robinson, "whether that be an NGO trying to conduct operations trying to provide food or water or support to the population."

Blackwater, headed by former U.S. Navy SEALs, was founded in part to take advantage of business opportunities created by the downsizing of the U.S. military.

The company is based at a 6,000-acre site in rural North Carolina, a campus the company calls "the most comprehensive private tactical training facility in the United States."

Security contractors in Iraq "are typically former special operations community personnel who are highly trained in the use of deadly force, also in surveillance detection and also in risk avoidance," Robinson said.

"I have heard estimates of up to 4,000 private sector security personnel working in Iraq," Crispin Hawes of the Eurasia Group, a research and consulting firm, said last year.

"Almost inevitably, these people are more exposed than those who tend to be protected," he said.

According to Brookings Institution statistics, 232 American civilian and security contractors have been killed in Iraq as of December 31.

Private American security contractors have also been killed in Gaza and Saudi Arabia in recent years.

Money is a key incentive for civilians to head into a war zone.

"It pays quite well. There's a lot of contracts that pay anywhere from $350 a day to $1,500 a day," said Chris Boyd of Kroll-Crucible Security.

Military analysts say the private security arrangement allows regular military troops to concentrate on fighting. But they are concerned that the lucrative pay offered by private contractors -- often more than $100,000 a year -- is depleting the ranks of the special forces.

This was a particularly bloody week for Western contract workers in Iraq. An American, a Canadian and an Australian were killed Wednesday on the route between the Green Zone and the airport in Baghdad and a Czech security contractor was killed during the weekend.

Six of the Blackwater employees killed Thursday were flying in a Bulgarian commercial helicopter. Three Bulgarian crew members and two Fijian security guards also were killed when the copter went down.

The Bulgarian Defense Ministry said missile fire downed the chopper, and U.S. military sources suspect that was the case. The chopper was between Baghdad and Tikrit when it went down.

Thursday's crash raises questions about the vulnerability in Iraq of commercial craft, both private planes and helicopters, which don't have the same defense capabilities as military aircraft.

There have been hopes that some of the transport work in Iraq could be shifted to commercial aircraft. Private charters and commercial aircraft fly in and out of Baghdad International Airport.